36 SCIENCE PROGRESS 



planetesimals of an identical average composition) could have 

 built up the earth, because in many respects (e.g. lack of 

 minerals which require the presence of mineralisers ; deficiency 

 in potash, etc.) the crustal rocks derived from them would have 

 been somewhat different in composition from those with which 

 we are familiar. In so far as the crust is the result of a long 

 and cumulative process of differentiation, it represents in a 

 highly magnified or exaggerated way, the detailed chemical 

 peculiarities of the materials from which the earth was formed. 



Summing up, we have seen that in the earth itself radium 

 and its congeners are undoubtedly more abundant in the upper 

 parts of the crust, and that in successive layers the radium 

 content rapidly decreases with depth. In meteorites, radium 

 is found in small quantities in the silicate minerals, but is 

 absent from the nickel-iron alloy. That is to say, the per- 

 centage of radium in each successive zone of the parent body 

 gradually decreased with depth, until ultimately it died out 

 altogether. It is suggested that, in the case of the earth, the 

 decrease of radium with depth does not stop at the point where 

 our means of observation come to an end, but that it continues 

 until at last the radium content is reduced to zero. If within 

 the depth to which the radio-elements extend, pressure and 

 temperature are ineffective in preventing or inhibiting atomic 

 decay, then the total quantity of the earth's store of the radio- 

 elements is calculable with some accuracy. 



The problems that are suggested by the general conclusions 

 of this inquiry are of supreme geological importance. The 

 evolution of the earth, of its zonal structure, and particularly 

 of its crust are all questions which remain to be solved. The 

 thermal history of the earth must be investigated afresh. 

 Volcanic phenomena, and the differentiation and movements 

 of molten magmas receive a new significance. To enter into 

 these wider problems l would lead us farther afield than the 

 title of this paper would justify, but its purpose will have been 

 served if it affords a basis for future discussion, and indicates the 

 special subjects concerning which the geologist urgently desires 

 extended knowledge and more securely founded conclusions. 



1 In this connection the following may be referred to : J. Joly, Radioactivity 

 and Geology, pp. 154-82, 1909; T. C. Chamberlin, Journ. Geol., p. 673, 191 1; 

 A. Holmes, Nature, p. 398, June 19, 1913, The Age of the Earth, p. 30, 1913 ; 

 L. L. Fermor, Geol. Mag., p. 65, 1914. 



